1. Field of the Invention
The instant invention is directed to a foamable and foamed composition and process for forming same. More particularly, the instant invention is directed to a foamable and foamed composition which includes an aromatic sulfone polymer, a linear polyester and an aromatic polycarbonate and a process of forming the compositions.
2. Background of the Prior Art
Of the engineering plastics, which are the high strength thermoplastics, the class of aromatic sulfone polymers is one of the most outstanding in terms of high temperature performance. That is, aromatic sulfone polymers can be utilized at elevated temperatures, above those at which other engineering plastics fail. This property suggests many unique uses for this resin. One such use, recently developed with the growth of microwave ovens, is food containers for prepackaged frozen food and the like. In the past frozen food packages were constructed of aluminum and other light weight metals. These packages easily withstood the elevated temperatures of standard gas or electrically heated cooking ovens. However, these metal containers cannot be used to heat foods contained therein by microwave energy. As those skilled in the art are aware, the high dielectric constant of metals result in a preferential absorption of the microwave energy by the metal covering the food preventing the heating effect of the microwave energy from penetrating to the food.
No such detriment exists when plastics are employed. Those skilled in the art are aware that plastics have very low dielectric constants. Microwave energy penetrates plastic packages without absorption so that the energy is focused on heating the foods contained therein. Thus, the use of a strong, high temperature resistant plastic such as aromatic sulfone polymers is suggested in this application.
The above analysis does not encompass two detrimental effects of using aromatic sulfone polymers. Engineering plastics, such as aromatic sulfones, are relatively high priced. In a mass market such as one involving preprepared foods, the high cost of packaging material is a critical factor. Equally significant in this application is handling of these packages. The criticality of lightweight packaging in handling the large numbers of prepackaged food containers involved in this application is readily apparent. Thus, the use of a conventional aromatic sulfone polymer package, in spite of the advantages noted above, is discouraged.
These detrimental factors can be overcome by using a foamed aromatic sulfone polymer. A foamed aromatic sulfone polymer is less dense. The same sized package could be produced at significantly decreased weight. At the same time the lower density of a foamed product decreases the aromatic sulfone polymer cost per package in direct proportion to the decrease in density of the foamed polymer compared to the density of the unfoamed product. However, prior success with foaming of sulfone polymers in the prior art would suggest abandonment of such a project.
As those skilled in the art are aware, typical foaming agents, often referred to as blowing agents, are usually toxic agents. The incorporation of these agents in food containers is therefore unacceptable. Even if non-toxic blowing agents were available which could pass stringent constraints, still the chemical blowing agents known in the art present serious processing conditions when added to engineering plastics. The addition of chemical blowing agents to engineering plastics require extreme processing conditions. For one thing, the presence of moisture results in degradation of the product produced. For another, chemical blowing agents are typically powders. In order to have a uniform foamed product, the foaming agent must be uniformly dispersed. To uniformly disperse a powder is a very difficult, and often, insoluble problem. Thus, the utilization of blowing agents oftentimes yields non-uniform foamed products.
While the above problems, associated with engineering plastics in general, are indeed formidable, they pale when foaming of an aromatic sulfone is attempted. Aromatic sulfone polymers, like other engineering plastics, are foamed by extruding the plastic with a foaming agent. The known chemical blowing agents, however, yield their gaseous product at relatively low temperatures. On the other hand, to extrude an aromatic sulfone requires a temperature in the range of about 250.degree. C. At atmospheric pressure, chemical blowing agents not only react to give off their gaseous product at temperatures significantly below 250.degree. C., resulting in premature foaming, but, more fundamentally, chemical blowing agents decompose at temperature below 250.degree. C. In sum, the known blowing agents of the prior art cannot be employed to foam aromatic sulfone polymers.
The foaming of engineering plastics other than aromatic sulfones is known in the prior art. Particularly, the foaming of polyethylene terephthalate is disclosed in U.S. Pat. No. 3,470,114, issued to Siggel et al. This patent is directed to a process for producing foamed polyethylene terephthalate by the addition thereto of an aromatic polycarbonate to which it is reacted at elevated temperature.
U.S. Pat. Nos. 4,462,947 and 4,466,933, each issued to the inventor of the present invention, discloses a similar product, a foamed polyethylene terephthalate formed by the reaction of polyethylene terephthalate with an aromatic polycarbonate to produce a foamed product. This product is subjected to crystallization annealing to produce lightweight products usable as food containers.
Although these teachings advance the art, those skilled in the art are aware of the clear superior high temperature properties of aromatic sulfones, compared to polyethylene terephthalate. However, the teachings of the prior art also suggest that making a foamed aromatic sulfone polymer presents such formidable problems that the substitution of aromatic sulfone polymer containers for polyethylene terephthalate containers, although desirable, is so difficult as to make such substitution highly unlikely.